Ok, great, thanks for the clarification.

Therefore a replica 3 configuration means raw storage space cost is 'similar' to a RAID 1 and actual data exists only 2 times and two different servers.


On 24/04/2017 11:35, Denis Chaplygin wrote:
With arbiter volume you still have a replica 3 volume, meaning that you have three participants in your quorum. But only two of those participants keep the actual data. Third one, the arbiter, stores only some metadata, not the files content, so data is not replicated 3 times.

On Mon, Apr 24, 2017 at 3:33 PM, FERNANDO FREDIANI <fernando.fredi...@upx.com <mailto:fernando.fredi...@upx.com>> wrote:

    But then quorum doesn't replicate data 3 times, does it ?


    On 24/04/2017 10:24, Denis Chaplygin wrote:

    On Mon, Apr 24, 2017 at 3:02 PM, FERNANDO FREDIANI
    <fernando.fredi...@upx.com <mailto:fernando.fredi...@upx.com>> wrote:

        Out of curiosity, why do you and people in general use more
        replica 3 than replica 2 ?

    The answer is simple - quorum. With just two participants you
    don't know what to do, when your peer is unreachable. When you
    have three participants, you are able to establish a majority. In
    that case, when two partiticipants are able to communicate, they
    now, that lesser part of cluster knows, that it should not accept
    any changes.

        If I understand correctly this seems overkill and waste of
        storage as 2 copies of data (replica 2)  seems pretty
        reasonable similar to RAID 1 and still in the worst case the
        data can be replicated after a fail. I see that replica 3
        helps more on performance at the cost of space.

    You are absolutely right. You need two copies of data to provide
    data redundancy and you need three (or more) members in cluster
    to provide distinguishable majority. Therefore we have arbiter
    volumes, thus solving that issue [1].


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